Memory system and semiconductor integrated circuit

ABSTRACT

A ferroelectric memory provided in a memory system stores in advance set data for data write time to memory cells. The set data include two types of data that differ between in a power-on state and in a power-off instruction time. When power is turned on, the set data that are stored in the ferroelectric memory are stored and retained in a latch circuit by a control circuit. Based on the set data retained in the latch circuit, data writing is performed in the ferroelectric memory respectively in the power-on state and in the power-off instruction time. Thus, operations of the ferroelectric memory can be controlled with desired operation timings according to operating conditions for each memory system. Excessive stress application to the ferroelectric memory during the power-on state is prevented and endurance deterioration is suppressed, while data retention characteristics after power-off are improved.

BACKGROUND OF THE INVENTION

The present invention relates to memory systems and semiconductorintegrated circuits and more particularly to memory systems andsemiconductor integrated circuits in which the operation timings can beset externally and which they can be operated at suitable timings thatsystems require.

Generally, semiconductor memories in a system set are incorporated on acommon board together with other semiconductor elements, such as D-Aconverters, A-D converters, CPUs, and control logic circuits. Likewise,in merged memory-logic devices (embedded memory devices), semiconductormemories are incorporated on a common chip together with microprocessorsand control logic circuits. In the former case, the semiconductormemories are formed in a package or in the form of bare chips, whereasin the latter case, they are formed using a common process for formingother devices that are incorporated on the chip.

In bare-chip-form semiconductor memories incorporated in system setproducts that are not packaged products, and in merged memory-logicproducts fabricated with control logic circuits using the same process,it is often the case that the control logic circuit also serves thefunction of a circuit for generating control signals to thesemiconductor memories, in order to reduce the area occupied by thecircuit. In that case, the control logic circuit having the dualfunction supplies memory control signals to semiconductor memories atpredetermined timings.

When constructing semiconductor memories on a common board or a commonsystem with microprocessors or the like, it is necessary to select andincorporate devices that can operate at an operating speed that isrequired for the memory in the set as a whole.

If a plurality of semiconductor memories are incorporated in a givensystem, however, the control logic circuit having the dual functionsupplies memory control signals to two or more semiconductor memories atthe same timings, and therefore, these semiconductor memories operatewith the same timings and consume electric current with the sametimings. Consequently, the system suffers from the drawback of largepeak currents.

In addition, when power supply voltages and memory ambient temperaturesof semiconductor memories varies, operation timings of the semiconductormemories change according to the variations, even though memory controlsignals are set to be output so that semiconductor memories in thesystem operate with suitable predetermined timings. Thus, the memoriessuffer from the drawback that timing deviations occur and the memoriesdo not operate with the suitable predetermined timings.

Such problems of the increase in peak current and the variation inoperation timings arise in merged memory-logic devices as well as insystem sets.

As described above, semiconductor memories and systems incorporatingthese have a drawback that operation timings are fixed irrespective ofoperating conditions, such as simultaneous operation with othersemiconductor memories, power supply voltages, and ambient temperatures.Similar drawbacks also exist in ferroelectric memories, which arenon-volatile memories. In the following, drawbacks in ferroelectricmemories (FeRAMs) are discussed.

First, deterioration of ferroelectric memory is discussed. When datawrite and rewrite are repeated in a ferroelectric memory, that is, whenpolarization reversals are repeated in a ferroelectric, theferroelectric suffers a ferroelectric fatigue deterioration phenomena,in which, for example, the repetition of polarization reversal causesremanent polarization to decrease. Because the ferroelectric memory is adestructive readout memory, the ferroelectric fatigue deteriorationphenomena occur as the ferroelectric undergoes polarization reversalsboth during write operation and during read operation, resulting inreliability problems, such as a decrease in data retention duration,readout incapability, and rewrite incapability. In the endurancedeterioration, which is one of the ferroelectric fatigue deteriorationphenomena, the degree of deterioration depends on the voltage applied tomemory cells during data writing and the time during which the voltageis applied, so the deterioration is promoted as the voltage is higherand the time during which the voltage is applied is longer whereas thedeterioration phenomenon is suppressed as the write voltage is lower andthe time during which the voltage is applied is shorter. However, insuch write operation, because the operation timings are predeterminedand the data write time is fixed, it is often the case that stress isexcessively applied to memory cells and thus ferroelectric fatiguedeterioration phenomena are promoted, which is undesirable in terms ofreliability.

Concerning prior art ferroelectric memories, Japanese Unexamined PatentPublication No. 3-113889, for example, discloses a technique ofsuppressing the endurance deterioration by reducing the voltage appliedto the memory cells during read operations. In addition, JapaneseUnexamined Patent Publication No. 3-5996 proposes a technique ofoperating a ferroelectric memory as a DRAM (volatile memory) duringnormal data-storing operations by switching the voltage applied to thememory cells during read operations between a high voltage and a lowvoltage. These conventional techniques, however, have such drawbacks asfollows. First, an increase in layout area is caused because multiplepower supply voltages need to be adopted in the semiconductor memory.Second, accuracy of applied voltages to memory cells degrades due tovariation in transistor performance, and voltage reduction effect withrespect to endurance deterioration vary between production lots. Third,readout defects easily occur due to shortage of the amount of readoutcharges.

SUMMARY OF THE INVENTION

It is an object to the present invention to configure operation timingsof a semiconductor memory to be variable according to the operatingconditions in a system set or a merged memory-logic device thatincorporates the semiconductor memory.

In order to accomplish the foregoing and other objects, according to thepresent invention, various kinds of operation timings of a semiconductormemory are stored beforehand, then one of the operation timings isselected according to operating conditions, and operations of thesemiconductor memory are controlled by the selected operation timing.

In accordance with one aspect, a memory system according to the presentinvention comprises: a non-volatile memory made of a plurality ofcircuit blocks operated by inputting a first signal; another memory madeof a plurality of circuit blocks operated by inputting a second signal;a data latch circuit retaining output timings and cycles of the firstsignal and output timings and cycles of the second signal; and a timinggenerating circuit outputting the first signal to the non-volatilememory and outputting the second signal to the other memory according tothe output timings and the cycles retained in the data latch circuit.

In the above-described memory system, the non-volatile memory may storethe output timings and the cycles of the first signal and output timingand cycle of the second signal beforehand and may comprises anothertiming generating circuit; and the other timing generating circuit maytransfer the output timings and the cycles of the first signal and theoutput timings and the cycles of the second signal stored in thenon-volatile memory to the data latch circuit when power is turned on.

In accordance with another aspect, the present invention provides amemory system comprising: a ferroelectric memory having a cell drivingblock and a data amplifying block, the cell driving block applyingvoltage to a data retention element and the data amplifying blockamplifying readout data from the data retention element; a data latchcircuit retaining output timings and pulse widths of control signalsrespectively controlling the cell driving block and the data amplifyingblock; and a timing generating circuit respectively outputting thecontrol signals to the cell driving block and to the data amplifyingblock according to the output timings and the pulse widths of thecontrol signals retained in the data latch circuit; wherein: the pulsewidths of the control signals retained in the data latch circuit are setso that the pulse widths are shorter in a power-on state during which apower supply potential is supplied to the ferroelectric memory, to thedata latch circuit, and to the timing generating circuit, whereas theyare longer in a power-off instruction time that is from the time whenthe cut-off of the power supply potential has been instructed until thesupply is cut off; and after the cell driving block and the dataamplifying block of the ferroelectric memory have been started tooperate with the output timings of the control signals, operatingperiods of the cell driving block and the data amplifying block are setto be longer in the power-off instruction time than in the power-onstate.

In the above-described memory system, the ferroelectric memory may carryout a data read operation and a data rewrite operation for a greaternumber of data retention elements within a single operation in thepower-off instruction time than in the power-on state.

In accordance with further another aspect, the present inventionprovides a memory system comprising: a ferroelectric memory having acell driving block and a data amplifying block, the cell driving blockapplying voltage to a data retention element and the data amplifyingblock amplifying readout data from the data retention element; a datalatch circuit retaining output timings and pulse widths of controlsignals respectively controlling the cell driving block and the dataamplifying block; and a timing generating circuit respectivelyoutputting the control signals to the cell driving block and to the dataamplifying block according to the output timings and the pulse widths ofthe control signals retained in the data latch circuit; wherein: thepulse widths of the control signals retained in the data latch circuitare set so that the pulse widths are shorter in a power-on state duringwhich a power supply potential is supplied to the ferroelectric memory,to the data latch circuit, and to the timing generating circuit, whereasthey are longer in a power-off instruction time that is from the timewhen the cut-off of the power supply potential has been instructed untilthe supply is cut off; and after the cell driving block and the dataamplifying block of the ferroelectric memory have been started tooperate with the output timings of the control signals, operatingperiods of the cell driving block and the data amplifying block are setto be longer in the power-off instruction time than in the power-onstate.

In the above-described memory system, the pulse widths of the controlsignals may be set to be longer when the temperature is low than whenthe temperature is high.

In accordance with yet another aspect, the present invention provides amemory system comprising: a ferroelectric memory having a cell drivingblock and a data amplifying block, the cell driving block applyingvoltage to a data retention element and the data amplifying blockamplifying readout data from the data retention element; a data latchcircuit retaining output timings and pulse widths of control signalsrespectively controlling the cell driving block and the data amplifyingblock; a timing generating circuit respectively outputting the controlsignals to the cell driving block and to the data amplifying blockaccording to the output timings and the pulse widths of the controlsignals retained in the data latch circuit; and a power supplypotential-detecting circuit detecting a power supply potential suppliedto the ferroelectric memory and outputting a selecting signalcorresponding to the detected power supply potential to the data latchcircuit; wherein: the data latch circuit retains, as the output timingsand the pulse widths of the control signals, a plurality of differentoutput timings and a plurality of different pulse widths that correspondto power supply potentials, and selects an output timing or a pulsewidth corresponding to the selecting signal from the power supplypotential-detecting circuit; and at least one of operation timings oroperating periods of the cell driving block and the data amplifyingblock of the ferroelectric memory is varied according to the powersupply potential supplied to the ferroelectric memory.

In the above-described memory system, the pulse widths of the controlsignals may be set longer when the power supply potential is low thanwhen the power supply potential is high.

In accordance with still another aspect, the present invention providesa memory system comprising: a ferroelectric memory; and a power supplypotential-supplying circuit receiving or being cut off from the supplyof a power supply potential from a power supply in response to anexternal signal, the power supply potential-supplying circuit supplyinga second power supply potential to the ferroelectric memory via a powersupply line; wherein the power supply potential-supplying circuit setsthe second power supply potential to be higher in a power-offinstruction time that is from the time when the cut-off of the powersupply potential from the power supply has been instructed until thesupply is cut off than in a power-on state during which the power supplypotential is supplied from the power supply.

In accordance with further another aspect, the present inventionprovides a memory system comprising: a ferroelectric memory having acell driving block and a data amplifying block, the cell driving blockapplying voltage to a data retention element and the data amplifyingblock amplifying readout data from the data retention element; a datalatch circuit retaining output timings and pulse widths of controlsignals respectively controlling the cell driving block and the dataamplifying block; a timing generating circuit respectively outputtingthe control signals to the cell driving block and to the data amplifyingblock according to the output timings and the pulse widths of thecontrol signals retained in the data latch circuit; a temperaturedetecting circuit detecting an ambient temperature and outputting aselecting signal corresponding to the detected temperature to the datalatch circuit; a power supply potential-detecting circuit detecting apower supply potential supplied to the ferroelectric memory andoutputting a selecting signal corresponding to the detected power supplypotential to the data latch circuit; and a power supplypotential-supplying circuit receiving or being cut off from the supplyof a power supply potential from a power supply in response to anexternal signal, the power supply potential-supplying circuit supplyinga second power supply potential to the ferroelectric memory via a powersupply line; wherein: the data latch circuit retains, as the outputtimings and the pulse widths of the control signals, a plurality ofdifferent output timings and a plurality of different pulse widths thatcorrespond to temperatures and power supply potentials, and selects anoutput timing or a pulse width corresponding to the selecting signalsfrom the temperature detecting circuit and the power supplypotential-detecting circuit; at least one of operation timings oroperating periods of the cell driving block and the data amplifyingblock of the ferroelectric memory is varied according to the powersupply potential supplied to the ferroelectric memory or the ambienttemperature; and the power supply potential-supplying circuit sets thesecond power supply potential to be higher in a power-off instructiontime that is from the time when the cut-off of the power supplypotential from the power supply has been instructed until the supply iscut off than in a power-on state during which the power supply potentialis supplied from the power supply.

The present invention further provides a semiconductor integratedcircuit in which one of the above-described memory systems as describedabove is incorporated on a single chip.

With the above-described configurations, the present invention achievesreduction in peak current in a system as a whole because such setting ispossible that the output timing and the cycle of the first signal outputto the non-volatile memory and the output timing and the cycle of thesecond signal output to another memory are different, and accordingly,the timings of current consumption in the non-volatile memory and in theother memory can be shifted from one another.

In particular, according to the present invention, the timing generatingcircuit that performs timing control necessary for the non-volatilememory to operate is incorporated in the non-volatile memory itself, andtherefore, even if signal racing occurs between the input signals fedfrom the memory system to the non-volatile memory, such signal racingcan be effectively prevented. Moreover, the next time the power isturned on, the FeRAM itself can read out required data using an internalcontrol circuit, and consequently, non-volatile memories for retainingdata after power-off can be eliminated.

Furthermore, according to the present invention, data write time to theferroelectric memory can be freely set from a device external of thememory by varying pulse widths of the control signals. Particularly inthe power-off instruction time, in order to sufficiently perform datawriting to the ferroelectric memory, voltage application to the dataretention element is set to be carried out for a longer time than in apower-on state, and therefore, data retention characteristics afterpower-off can be improved.

Further according to the present invention, in the power-off instructiontime, more data retention elements are subjected to a rewrite operation(refresh operation) than in the power-on state, and therefore, datawrite in the power-off instruction time can be performed within a shorttime.

In addition, according to the present invention, data write time to theferroelectric memory can be changed according the temperature of thememory system surrounding the ferroelectric memory so that, when thetemperature is low, the pulse widths of the control signals given to thecell driving block and to the data amplifying block are set to be longerthan those when the temperature is high. Therefore, data retentioncharacteristics are improved since excessive stress to the ferroelectricmemory can be prevented and voltage application time can be optimized.

Moreover, according to the present invention, when the power supplypotential supplied to the ferroelectric memory decreases because of thevariation in power supply potential caused by device operations in thesystem or the variation in power supply potential resulting from thepower source, pulse widths of the control signals given to the celldriving block and to the data amplifying block in the ferroelectricmemory are set to be longer than those when the power supply potentialis high. Therefore, data retention characteristics are improved sincedata write time to the ferroelectric memory is optimized and datawriting (causing polarization in data retention elements in theferroelectric memory) is sufficiently performed.

Still further, according to the present invention, the power supplypotential supplied to the ferroelectric memory is shifted higher in thepower-off instruction time than in the power-on state. Therefore, fasterdata rewrite operations can be achieved in the power-off instructiontime, and data retention characteristics can be improved since datawriting to the ferroelectric memory is sufficiently carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the general configuration of a memory system according toEmbodiment 1 of the present invention.

FIG. 2 shows the internal configuration of a timing generating circuitprovided in the memory system.

FIG. 3 illustrates operation timings of a ferroelectric memory and avolatile memory comprised in the memory system.

FIG. 4 illustrates data read operation timings of the ferroelectricmemory in a power-on state.

FIG. 5 illustrates data read operation timings of the ferroelectricmemory in a power-off instruction time.

FIG. 6 shows the general configuration of a memory system according toEmbodiment 2 of the present invention.

FIG. 7 shows a characteristic of time required for data writing to theferroelectric memory with respect to system temperature.

FIG. 8 shows a characteristic of time required for data writing to theferroelectric memory with respect to a power supply potential.

FIG. 9 shows the internal configuration of a timing generating circuitprovided in the memory system.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are detailed below withreference to the attached drawings.

Embodiment 1

FIG. 1 schematically shows the configuration of a memory system inaccordance with Embodiment 1 of the present invention. The memory systemshown in the figure is a chipset in which a plurality of chips are puttogether, or a system LSI (semiconductor integrated circuit) which isincorporated in a single chip, and is made up of a ferroelectric memory10, which is a non-volatile memory, a logic circuit 14, and anothervolatile memory 20 provided separately from the memory 10, such as anSRAM. The separately provided memory 20 is not limited to a volatilememory but may be a non-volatile memory.

The ferroelectric memory 10 has a memory address-assigning circuit 1, adata read circuit (cell driving block) 2, a data amplifying/writingcircuit (data amplifying block) 3, a data input/output circuit 4, and acontrol circuit 9 that outputs control signals 5, 6, 7, and 8respectively to these circuits 1 to 4. The memory address-assigningcircuit 1 carries out, for example, operations up to word line selectionaccording to external input addresses, and the data read circuit 2carries out voltage application (driving of cell plate lines) to memorycells (data retention elements) in order to read out data fromferroelectric memory cells. The data amplifying/writing circuit 3amplifies the very small electric charge that is read out from memorycells by the operation of the data read circuit 2 and transfers it tothe data input/output circuit 4. The data input/output circuit 4 outputsthe amplified data and takes in externally-supplied input data.

The control circuit (another timing generating circuit) 9 has a functionof detecting and controlling the operation sequence of theabove-described circuits (hereafter referred to as “circuit blocks”) 1to 4. When control signals 12 from a timing generating circuit 13, whichwill be detailed later, are not used, such as when power is turned on,the control circuit 9 also serves a function of controlling fundamentaloperations of the circuit blocks 1 to 4 that are necessary for thenormal operation of the ferroelectric memory 10.

The volatile memory 20 has a memory address-assigning circuit 15, a dataread/write circuit 16, a data input/output circuit 17, and a controlcircuit 19 that outputs control signals 18 to these circuit blocks 15 to17.

In addition, the logic circuit 14 has a latch circuit 11 and a timinggenerating circuit 13 that generates an input signal (control signal) 12supplied to the control circuit 9 of the ferroelectric memory 10 and acontrol signal 21 supplied to the control circuit 19 of the volatilememory 20. During the time when the power is on, the latch circuit (datalatch circuit) 11 retains the data that has been read out from theferroelectric memory 10 when power has been turned on. Based on the dataretained in the latch circuit 11, the timing generating circuit 13changes output timings and cycles of the control signal (first signal)12 supplied to the ferroelectric memory 10 and of the control signal(second signal) 21 supplied to the volatile memory 20 according tooperating conditions, to change operation timings and operation cyclesof the ferroelectric memory 10 and the volatile memory 20.

The internal configuration of the timing generating circuit 13 of thelogic circuit 14 is shown in FIG. 2. This figure shows, as an example,only the portion of the timing generating circuit 13 that is concernedwith the ferroelectric memory 10, and omits the portion that isconcerned with the volatile memory 20, as the configurations of theseportions are the same. In the figure, reference numeral 13 a denotesselector switches and reference numeral 13 b delay circuits, and each ofthe selector switches 13 a is provided between two delay circuits 13 b.In addition, a code circuit 13 c that controls a plurality of selectorswitches 13 a (three switches in the case of the figure) is arranged inthe timing generating circuit 13. The code circuit 13 c controls theplurality of selector switches 13 a according to timing data D0 and D1and inverted data/D0 and /D1, which are inverted data of D0 and D1, thatare received from the latch circuit 11. For example, during a power-onstate in which the data D0 and D1 are (1, 1), an input path 13 d of thecontrol signal 12 is connected to an output path 13 e only through theselector switch 13 a that is positioned on the left side of the figure.On the other hand, at power-off instruction time in which the data D0and D1 are (0, 0), the input path 13 d is connected to the output path13 e by the selector switch 13 a that is positioned on the right side ofthe figure so that the control signal 12 is delayed through two delaycircuits 13 b and is then output.

Next, an operation of the memory system having the above-describedconfiguration is detailed below. Here, a specific example of the controlsignal 12 is described with reference to FIG. 3. The following describesa case in which the logic circuit 14 controls the internal operation ofthe ferroelectric memory 10 using signals that are compatible with thevolatile memory (SRAM) 20.

In FIG. 3, /WE denotes a signal for identifying a read operation and awrite operation, CP denotes a signal for starting data-reading frommemory cells, /OE denotes a signal for controlling the start and thestop of data output, and SAE denotes a signal for controlling the startand the stop of amplifying operation for the readout data. The figure isan explanatory diagram showing the case when operation timings arevaried between the ferroelectric memory 10 and the volatile memory 20.Specifically, the rise and fall timings and the cycles of the signal/WE, the signal CP, the signal /OE, and the signal SAE are not made thesame between the ferroelectric memory 10 and the volatile memory 10, butthey are set to have different timings and cycles. The rise and falltimings of these four types of signals /WE, CP, /OE, and SAE for each ofthe memory 10 and the memory 20 are stored beforehand in theferroelectric memory 10, are read out with the control circuit 9 fromthe ferroelectric memory 10 when power is turned on, and are transferredto the latch circuit 11 to be stored therein.

Thus, in the present embodiment, as shown in FIG. 3, the peak values ofthe current consumption of the ferroelectric memory 10 and the volatilememory 20 are distributed over time as indicated by the solid line inthe figure, and the current consumption does not converge as in the caseindicated by the dash-dotted line in the figure, in which the volatilememory 20 operates at the same timings as the ferroelectric memory 10.Therefore, the peak current value of the system as a whole can besuppressed to a small value.

The timing data of the timing control signals /WE, CP, /OE, and SAE forthe ferroelectric memory 10 and the volatile memory 20 are stored andset in the ferroelectric memory 10 for each system. These timing datacan be rewritten as needed since they are stored in the ferroelectricmemory 10, so peak current can be suppressed on a system-by-systembasis.

The foregoing discussion concerns only with the read operation, but thewrite operation may be handled in a similar manner. Therefore, accordingto the present embodiment, the peak current of the system as a whole canbe suppressed to low levels both during the read operation and duringthe write operation.

Next, the operation of the memory system according to the presentembodiment is described for the period from the time when power isturned on, during which power is on, and up to the power-off instructiontime.

When power is turned on, under the control of the control circuit 9, theaddress-assigning circuit 1 selects a specific address, and operationtiming data stored in the corresponding memory cells are read out,amplified, and then transferred from the data input/output circuit 4 tothe latch circuit 11, in which they are stored and retained therein.These operation timing data are the operation timing data of theferroelectric memory 10 for the following two periods: a period from thetime when power-on has been completed until a power-off instruction isreceived from the outside (power-on state), and a period from the timewhen the power-off instruction has been received from the outside untilthe power supply is actually cut off (power-off instruction time).

In the power-on state, the timing generating circuit 13 of the logiccircuit 14 outputs the control signal 12 with an operation timing and apulse width that correspond to the operation timing data for power-onstate that is stored in the latch circuit 11. In the following, aspecific example of the control signal 12 is described with reference toFIG. 4. In FIG. 4, /CE denotes a signal for controlling the start andthe stop of the operation, WL denotes a word line-selecting signal, /WEdenotes a signal for identifying a read operation and a write operation,CP denotes a signal for starting a data-reading from memory cells, /OEdenotes a signal for controlling the start and the stop of data output,and SAE denotes a signal for controlling the start and the stop of theamplifying operation for the readout data.

Referring to FIG. 4, at the time indicated by reference character A, anaddress is taken in with the fall of the control signal /CE of the logiccircuit 14, and the operation up to the selection of a word line iscarried out. Subsequently, at the time indicated by reference characterB, the signal CP rises with the fall of the signal /WE of the logiccircuit 14, and a data read operation from memory cells starts. Then, atthe time indicated by reference character C, the signal SAE starts upwith the fall of the signal /OE of the logic circuit 14, and a senseamplifier starts a read data-amplifying operation. In the ferroelectricmemory 10, a start period I, a read period II, and a write period III,as shown in the figure, can be defined using the three signals /CE, /WE,/OE. As shown in the figure, the total of a period (4) from referencecharacter C to reference character D and a period (5) from referencecharacter D to reference character E, that is, a period represented as(4)+(5), corresponds to a data write period. Thus, data write periodscan be freely set by controlling a period from the fall of the signal/OE until the rise of the signal /WE and a period from the rise of thesignal /WE until the rise of the signal /OE. For this reason, in thepresent embodiment, timing data for the signal /WE and the signal /OEare different between the power-on state and the power-off instructiontime. More details are given below.

In the power-on state, data write time is shortened to prevent endurancedeterioration of the ferroelectric memory 10. By contrast, in thepower-off instruction time, the data write time is increased in order toobtain better data retention characteristics after power-off. That is,as will be understood from the comparison between FIG. 4 and FIG. 5,which illustrate the timings in a power-on state and the timings in apower-off instruction time, respectively, the timing in the power-offinstruction time, depicted in FIG. 5, is such that each of the period(4) and the period (5) is set to be long. Specifically, although thefall timing of the signal /OE, indicated by reference character C, isunchanged, the period (pulse width) (4) is set longer by delaying therise timing of the signal /WE, indicated by reference character D, incomparison with that shown in FIG. 4, and at the same time, the period(pulse width) (5) is set longer by delaying the rise timing of thesignal /OE in comparison with that shown in FIG. 4. Thus, it is possibleto increase the data write time to the ferroelectric memory 10 in thepower-off instruction time.

Thus, in the present embodiment, the operation timing data for thepower-on state and for the power-off instruction time of theferroelectric memory 10 are stored beforehand in the ferroelectricmemory 10, and these data are read out when power is turned on and thentransferred to the latch circuit 11 in which the data are stored andretained. Consequently, in the power-on state, because the data writetime is short, it is possible to prevent development of a ferroelectricfatigue deterioration phenomenon that is caused by excessive stress tothe memory cells of the ferroelectric memory 10, and to improve devicereliability. On the other hand, in the power-off instruction time inwhich a power-off instruction is input from the outside of the memorysystem, it is possible to improve data retention characteristics afterpower-off because the data write time is long.

In the power-off instruction time, the ferroelectric memory 10 carriesout a refresh operation for all the memory cells. This contributes toenhancement of the speed of the sequence in the power-off instructiontime since the number of memory cells that can be rewritten per cycleincreases.

Moreover, even if signal racing occurs between the control signals inputfrom the timing generating circuit 13 of the logic circuit 14, defectsin operation timings of the ferroelectric memory 10 and the volatilememory 20 resulting from such signal racing are reliably prevented fromoccurring because the ferroelectric memory 10 and the volatile memory 20incorporate the control circuits 9 and 19, respectively, and each ofthese control circuits 9 and 19 has a function of detecting theoperation sequence of the corresponding memory.

In the present embodiment, the switching of operation timings in theferroelectric memory 10 and the control of the change of operationtimings between the ferroelectric memory 10 and the volatile memory 20are performed in the system by providing the logic circuit 14 outsidethe ferroelectric memory 10, but it is of course also possible toconfigure these functions to be provided and carried out in theferroelectric memory 10.

In addition, in the present embodiment, the logic circuit 14 includesthe latch circuit 11 for retaining, during the power-on state, operationtiming data concerning the data write time that are read out from theferroelectric memory 10 when power is turned on, and the logic circuit14 is arranged outside the ferroelectric memory 10. However, it is ofcourse also possible to obtain similar effects even when these functionsare provided within the ferroelectric memory 10.

Embodiment 2

Next, a memory system according to Embodiment 2 of the present inventionis described below with reference to the drawings.

FIG. 6 schematically shows the configuration of a memory systemaccording to Embodiment 2 of the present invention. The memory systemshown in the figure has the ferroelectric memory 10 and the logiccircuit 14 of the memory system shown in FIG. 1, but does not have thevolatile memory 20.

The ferroelectric memory 10 receives power supply from a power supplypotential-supplying circuit 26. The power supply potential-supplyingcircuit 26 is, in response to an external signal, supplied with a powersupply potential from a power supply or cut off from the supply thereof,and while the power supply potential is being supplied, it adjusts thereceived power supply potential and supplies the adjusted power supplypotential as a second power supply potential to devices in the memorysystem, which include the ferroelectric memory 10, via a power supplyline 28.

Further, the memory system shown in FIG. 6 is provided with a powersupply potential-detecting circuit 24 that detects the second powersupply potential supplied from the power supply potential-supplyingcircuit 26, and a temperature detecting circuit 22 that detects thetemperature (ambient temperature) of the memory system. The temperaturedetecting circuit 22 monitors, for example, variation in transistorcharacteristics due to temperature variation of the memory system todetect a temperature deviation from room temperature, and generates atiming data-selecting signal 23 that corresponds to the deviation. Thepower supply potential-supplying circuit 24 detects variations in thesecond power supply potential resulting from device operations in thememory system by monitoring deviations in voltage value from a referencepotential or deviations in current value from a reference current todetect deviations in the second power supply potential from a referencepower supply potential, and generates a timing data-selecting signal 25that corresponds to the deviation. Both the selecting signal 23 from thetemperature detecting circuit 22 and the selecting signal 25 from thepower supply potential-detecting circuit 24 are output to the latchcircuit 11 of the logic circuit 14.

The ferroelectric memory 10 stores in advance a characteristic of timerequired for data writing to the ferroelectric memory 10 with respect totemperature of the memory system, as shown in FIG. 7, and acharacteristic of time required for data writing to the ferroelectricmemory 10 with respect to the second power supply potential suppliedfrom the power supply potential-supplying circuit 26, as shown in FIG.8. Both of these characteristics are read by the control circuit 9 whenpower is turned on, and then they are stored and retained in the latchcircuit 11. The characteristic of time required for data writing withrespect to temperature, shown in FIG. 7, is set so that when thetemperature is high, the time required for data writing is short,whereas when the temperature is low, the time required for data writingis long. Likewise, the characteristic of time required for data writingwith respect to the second power supply potential, shown in FIG. 8, isset so that when the power supply potential is high, the time requiredfor data writing is short, whereas when the power supply potential islow, the time required for data writing is long.

The latch circuit 11 of the logic circuit 14 receives the selectingsignal 23 from the temperature detecting circuit 22, selects a timerequired for data writing that corresponds to the temperature of thememory system according to the characteristic shown in FIG. 7, andoutputs it to the timing generating circuit 13. The latch circuit 11also receives the selecting signal 25 from the power supplypotential-detecting circuit 24, selects a time required for data writingthat corresponds to the second power supply potential supplied to thememory system according to the characteristic shown in FIG. 8, andoutputs it to the timing generating circuit 13.

In the timing generating circuit 13, the numbers of the selectorswitches 13 a and the delay circuits 13 b are restricted in order toreduce layout area. If, for example, because of this restriction, thetiming generation corresponding to variation in the second power supplypotential caused by device operations in the memory system and tovariation in the system temperature cannot be achieved with the scale ofthe elements provided internally, timing data D0 and D1 are set to be(0, 1), as shown in FIG. 9, to switch the selector switch 13 a that ispositioned in the middle of FIG. 9 so that the input path 13 d isconnected to the output path 13 e only through the delay circuit 13 bthat is positioned on the left side of the figure, and additionally, thecontrol signal is set to be “1”, for example. Further, the timinggenerating circuit 13 sets the control signal 27 supplied to the powersupply potential-supplying circuit 26 to be “1” also in the power-offinstruction time.

The power supply potential-supplying circuit 26 has a function ofshifting the second power supply potential that is output to the powersupply line 28 thereof to a higher level when it receives the controlsignal 27 that is “1” from the timing generating circuit 13.

Thus, in the present embodiment, when power is turned on, thecharacteristics of time required for data writing shown in FIGS. 7 and 8are read out from the ferroelectric memory 10 and are then stored in thelatch circuit 11.

Thereafter, according to temperature variation of the memory system, thetemperature detecting circuit 22 outputs a selecting signal 23corresponding to the temperature of the memory system to the latchcircuit 11, and the latch circuit 11 selects a time required for datawriting corresponding to the temperature of the memory system accordingto the characteristic shown in FIG. 7, and the timing generating circuit13 controls data writing into the ferroelectric memory 10 via thecontrol circuit 9 based on the selected time required for data writing.As a consequence, even when the memory system is in high temperatures,device reliability can be improved because excessive stress to theferroelectric memory 10 is prevented and voltage application time isoptimized by reducing the time required for data writing.

In addition, when the second power supply potential supplied from thepower supply potential-supplying circuit 26 to the memory system varies,the power supply potential-detecting circuit 24 the variation andoutputs the selecting signal 25 that corresponds to the varied powersupply potential to the latch circuit 11. Based on the selecting signal25, the latch circuit 11 selects a time required for data writingcorresponding to the varied second power supply potential according tothe characteristic shown in FIG. 8, and the timing generating circuit 13controls data writing in the ferroelectric memory 10 via the controlcircuit 9 based on the selected time required for data writing. As aconsequence, even when the second power supply potential supplied to thememory system varies, device reliability can be effectively improvedbecause excessive stress to the ferroelectric memory 10 is prevented andvoltage application time is optimized.

Further, even if the timing generating circuit 13 cannot set a timingcorresponding to the time required for data writing because ofrestrictions in terms of the scale of the elements, although the latchcircuit 11 outputs an appropriate time required for data writingaccording to variation in the system temperature or variation in thepower supply potential, the timing generating circuit 13 outputs thecontrol signal 27 that is “1” so that the power supplypotential-supplying circuit 26 shifts the second power supply potentialsupplied to the power line 28 to a higher level. As a consequence, inthe ferroelectric memory 10, good data writing can be performed withhigh power supply potentials even if a pulse width corresponding to atime required for data writing is not obtained.

Furthermore, according to the present embodiment, in the power-offinstruction time, the timing generating circuit 13 outputs the controlsignal 27 that is “1” and the second power supply potential suppliedfrom the power supply potential-supplying circuit 26 to theferroelectric memory 10 is shifted to a higher level. This contributesto faster data writing and to improvement in data retentioncharacteristics as well as to the setting of a long data write time inthe power-off instruction time

It should be noted that, although the temperature detecting circuit 22,the power supply potential-detecting circuit 24, and the power supplypotential-supplying circuit 26 are arranged outside the ferroelectricmemory 10 in the present embodiment, it is of course possible to providethese circuits within the ferroelectric memory 10.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1-8. (canceled)
 9. A memory system comprising: a ferroelectric memory;and a power supply potential-supplying circuit receiving or being cutoff from the supply of a power supply potential from a power supply inresponse to an external signal, the power supply potential-supplyingcircuit supplying a second power supply potential to the ferroelectricmemory via a power supply line; wherein the power supplypotential-supplying circuit sets the second power supply potential to behigher in a power-off instruction time that is from the time when thecut-off of the power supply potential from the power supply has beeninstructed until the supply is cut off than in a power-on state duringwhich the power supply potential is supplied from the power supply.10-11. (canceled)